1. Field of the Invention
The present invention relates to an isolated DC-DC converter that has a configuration for indirectly detecting an output voltage supplied to the exterior and performing stabilizing control of the output voltage on the basis of the detected voltage.
2. Description of the Related Art
FIG. 6 shows main circuit components of a typical isolated DC-DC converter. The isolated DC-DC converter 1 includes a transformer 2. A main switching device (for example, a MOS-FET) Q and an input filter circuit 3 are provided on the side of a primary coil N1 of the transformer 2. Energy is supplied to the primary coil N1 from an external power supply 4 via the input filter circuit 3 by the switching operation of the main switching device Q.
A secondary-side rectifying and smoothing circuit 5 is provided on the side of a secondary coil N2 of the transformer 2. The secondary-side rectifying and smoothing circuit 5 includes a rectification-side synchronous rectifier (for example, a MOS-FET) 6, a commutation-side synchronous rectifier (for example, a MOS-FET) 7, a synchronous-rectifier driving circuit 8, and a smoothing circuit 9. The voltage output from the secondary coil N2 corresponds to the voltage generated in the primary coil N1. The secondary-side rectifying and smoothing circuit 5 rectifies and smoothes the output voltage from the secondary coil N2 to produce a direct-current voltage and outputs the direct-current voltage to an external load S as an output voltage Vout.
A tertiary-side rectifying and smoothing circuit 10 is provided on the side of a tertiary coil N3 of the transformer 2. The tertiary-side rectifying and smoothing circuit 10 includes a rectification-side diode 11, a commutation-side diode 12, a choke coil 13, a smoothing capacitor 14, and voltage-dividing resistors 15 and 16. The tertiary-side rectifying and smoothing circuit 10 rectifies and smoothes the output voltage from the tertiary coil N3 to produce a direct-current voltage and detects and outputs the direct-current voltage as a detected voltage Vk of the output voltage Vout from the secondary-side rectifying and smoothing circuit 5.
The isolated DC-DC converter 1 further includes an error amplifier 18. The error amplifier 18 outputs a voltage corresponding to the difference between the detected voltage Vk output from the tertiary-side rectifying and smoothing circuit 10 and a reference voltage Vs from a reference supply 17. The isolated DC-DC converter 1 further includes a control circuit 20. The control circuit 20 has circuitry for controlling the switching operation of the main switching device Q by, for example, the PWM control method on the basis of the output voltage from the error amplifier 18 (i.e., on the basis of the detected voltage Vk from the tertiary-side rectifying and smoothing circuit 10) so that the output voltage Vout from the secondary-side rectifying and smoothing circuit 5 is stabilized at a predetermined voltage. In this example, the control circuit 20 uses a direct-current voltage Vcc output from the smoothing capacitor 14 of the tertiary-side rectifying and smoothing circuit 10 as a supply voltage.
Patent Document 1: Japanese Patent No. 3391320.
Patent Document 2: Japanese Patent No. 3339452.
In the aforementioned isolated DC-DC converter 1, it is desirable that the output voltage Vout be completely proportional to the detected voltage Vk output from the tertiary-side rectifying and smoothing circuit 10 to achieve satisfactory accuracy of the output voltage. However, in the configuration of the isolated DC-DC converter 1 shown in FIG. 6, there is a problem such that the proportional relationship between the output voltage Vout and the detected voltage Vk is broken due to the circuit operation that is described below in the period in which the main switching device Q is switched off.
An example of the circuit operation in the period in which the main switching device Q is switched off will now be described using the wave form chart of FIG. 7. For example, when the main switching device Q is switched off (time t0), LC resonance due to parasitic capacitance generated in parallel between the source and drain of the main switching device Q and excitation inductance of the transformer 2 begins. This generates a pulse voltage of the LC resonance as shown in FIG. 7 at the drain of the main switching device Q. When a half cycle of the LC resonance has elapsed (time t1), resetting of the transformer 2 is completed.
The drain voltage of the main switching device Q is in a state in which the drain voltage is clamped at a voltage Vd described below during the period between the time when resetting of the transformer 2 is completed and the time when the main switching device Q is turned on (the period between time t1 and time t2). Moreover, a driving voltage is applied to the gate of the commutation-side synchronous rectifier 7 by the synchronous-rectifier driving circuit 8 so that the commutation-side synchronous rectifier 7 is controlled so as to be in an on-state during the period in which the main switching device Q is switched off. Moreover, no driving voltage is applied to the gate of the rectification-side synchronous rectifier 6 so that the rectification-side synchronous rectifier 6 is controlled so as to be in an off-state during the period in which the main switching device Q is switched off.
Energy due to excitation inductance of a choke coil (not shown) that defines a smoothing circuit 9 is applied along a path A as shown in FIG. 6 so that power is supplied to the load S during the period in which the main switching device Q is switched off. The rectification-side synchronous rectifier 6 is controlled so as to be in an off-state as described above during the period in which the main switching device Q is switched off. However, due to a parasitic diode generated in parallel between the drain and source of the rectification-side synchronous rectifier 6, an excitation current of the transformer 2 circulates around a path through the secondary coil N2 of the transformer 2, the commutation-side synchronous rectifier 7, the parasitic diode of the rectification-side synchronous rectifier 6, and the secondary coil N2 when resetting of the transformer 2 is completed. This generates a forward drop-down voltage Vf of the parasitic diode across both ends of the rectification-side synchronous rectifier 6. Thus, the voltage at both ends of the secondary coil N2 is clamped at the forward drop-down voltage Vf of the parasitic diode of the rectification-side synchronous rectifier 6 during the period between the time when resetting of the transformer 2 is completed and the time when the main switching device Q is turned on (the period between t1 and t2 (transformer-excitation-current circulation period)).
Accordingly, in a case where Vin is an input voltage supplied from the external power supply 4 to the isolated DC-DC converter 1, N1 is the number of turns of the primary coil N1, N2 is the number of turns of the secondary coil N2, and N3 is the number of turns of the tertiary coil N3, a clamp voltage Vd of the drain of the main switching device Q during the transformer-excitation-current circulation period (the period between t1 and t2) is calculated by an expression Vd Vin−(N1/N2)×Vf. A voltage V3 generated in the tertiary coil N3 is clamped at a voltage calculated by an expression V3=(N3/N2)×Vf.
In the tertiary-side rectifying and smoothing circuit 10, current is applied along a path B that passes through the choke coil 13 and the commutation-side diode 12 as shown in FIG. 6 due to energy stored in the choke coil 13 during the period in which the main switching device Q is switched off. The voltage V3 is generated in the tertiary coil N3 as described above during the period in which the main switching device Q is switched off. In the tertiary-side rectifying and smoothing circuit 10, the diode 12 having one-way conductivity is provided as a rectifying device on the commutation side. Thus, current due to the voltage V3 of the tertiary coil N3 does not follow a path that sequentially passes through the commutation-side diode 12 and the rectification-side diode 11 but follows a path C that passes through the choke coil 13, the rectification-side diode 11, and the tertiary coil N3, as shown in FIG. 6. In the tertiary-side rectifying and smoothing circuit 10, the detected voltage Vk during the period in which the main switching device Q is switched off is obtained by superimposing a voltage caused by applying current along the path B on a voltage caused by applying current along the path C.
During the aforementioned period in which the main switching device Q is switched off, the voltage Vout output from the secondary-side rectifying and smoothing circuit 5 to the load S is not affected by the voltage Vf generated in the secondary coil N2. In contrast, the detected voltage Vk output from the tertiary-side rectifying and smoothing circuit 10 is affected by the voltage V3 of the tertiary coil N3 due to the voltage Vf of the secondary coil N2. Thus, the correlation between the output voltage Vout from the secondary-side rectifying and smoothing circuit 5 and the detected voltage Vk from the tertiary-side rectifying and smoothing circuit 10 is broken.
That is to say, the correlation between the detected voltage Vk from the tertiary-side rectifying and smoothing circuit 10 and the output voltage Vout from the secondary-side rectifying and smoothing circuit 5 is weakened by a voltage V2 given by the following expression:V2=Vf×(N3/N2)×(Tcy/Tsw).
Here, Vf is a forward drop-down voltage of the parasitic diode of the rectification-side synchronous rectifier 6 during the period in which the main switching device Q is switched off, N2 is the number of turns of the secondary coil N2, N3 is the number of turns of the tertiary coil N3, Tcy is the length of the transformer-excitation-current circulation period, and Tsw is the length of one switching cycle.
In the isolated DC-DC converter 1 having the circuitry shown in FIG. 6, the length of the transformer-excitation-current circulation period depends on the magnitude of the input voltage Vin. Thus, a change in the input voltage Vin changes the relationship between the output voltage Vout from the secondary-side rectifying and smoothing circuit 5 and the detected voltage Vk from the tertiary-side rectifying and smoothing circuit 10. The forward drop-down voltage Vf of the diode increases as the environmental temperature becomes low and decreases as the environmental temperature becomes high. Accordingly, the relationship between the output voltage Vout from the secondary-side rectifying and smoothing circuit 5 and the detected voltage Vk from the tertiary-side rectifying and smoothing circuit 10 is changed by a change in the environmental temperature.
In this way, the relationship between the output voltage Vout and the detected voltage Vk is changed by a change in the input voltage Vin and a change in the environmental temperature. Thus, it is quite difficult to correct the detected voltage Vk so that the detected voltage Vk is proportional to the output voltage Vout. That is to say, in the circuitry of the isolated DC-DC converter 1 shown in FIG. 6, it is quite difficult to achieve a completely proportional relationship between the output voltage Vout and the detected voltage Vk, and there is a problem such that satisfactory accuracy of the output voltage. Vout cannot be achieved. In particular, the ratio of the number of turns of the tertiary coil N3 to the number of turns of the secondary coil N2 (N3/N2) has tended to increase recently. Accordingly, the correlation between the output voltage Vout and the detected voltage Vk has been weakened, and it is increasingly difficult to hold the range of variation in the output voltage Vout within a predetermined tolerance in an isolated DC-DC converter 1 that has a low output voltage Vout and is low-powered.